The gravitational force of attraction between two massive bodies is the force which results from the inherent natural attraction between the two bodies. The magnitude of the gravitational force is directly related to the mass of the bodies and is inversely related to the square of the distance between the centers of mass of the two bodies. Gravity is measured as acceleration. For instance, the free-fall acceleration due to gravity near the earth's surface of an object having a small mass compared to the mass of the earth is about 9.8 m/s2.
A gravimeter is an instrument used to measure the strength or magnitude of gravity. Gravimeters are well known and typically measure the vertical component of the total gravity vector of the earth in units of acceleration at a particular location. The common unit of measurement of gravity is the “gal.” A gal is a unit of acceleration defined as 1 cm/s2=0.01 m/s2≈10−3 g. These types of measurements are referred to herein as “gravity measurements.”
Gravity measurements are useful for many purposes, as illustrated by the following examples. Gravity measurements are used to monitor subsurface density changes resulting from immediate to long term subterranean events. Gravity measurements are also used to monitor the influx of water when flooding a petroleum reservoir to push hydrocarbons into extraction wells. Any movement of waste gas and liquid substances stored in subsurface caverns or containments can be monitored by gravity measurements to detect whether the waste and liquid substances remain securely confined. Water management techniques make use of gravity measurements to monitor the extent to which groundwater moves or the extent to which rainwater penetrates into and saturates the soil.
In all of these uses and others, changes in the quantity of the monitored substance, such as the oil, water or gas, alter the density of the volume of mass at the monitored location. That change in mass, through Newton's law, changes the gravity around and above that specific monitored location. For example, in the absence of any other change, the depletion of petroleum from a subterranean reservoir decreases gravity at the location above the reservoir due to the reduction of petroleum in the reservoir. Similarly, flooding a petroleum reservoir with water increases gravity above the reservoir because the water replaces a less dense substance or fills a void. Dynamic effects may also be determined using gravity measurements. For example the movement of groundwater and waste substances from their previous locations creates temporal changes in gravity which may be sensed. The extent of movement of the substance can be determined from gravity measurements, and with appropriate accuracy, the volumetric quantities of the moving substance can also be determined.
The change in mass of the monitored substance is normally very small compared to the mass of the surrounding earth that defines the reservoir, cavern or confinement of the substance. Consequently, the change in gravity is usually very small. Nevertheless, the change does occur and gravimeters are capable of measuring such relatively small changes in gravity.
Gravimeters fall into two categories: a relative gravity measurement instrument known as a relative gravimeter, and an absolute gravity measurement instrument known as an absolute gravimeter. Both types of gravimeters measure the vertical component of the earth's total gravity vector. Gravimeters are distinguished from another type of measurement instrument known as a gradiometer. A gradiometer is used to measure a gradient, differential, difference or rate of change of gravity. Gradiometers are therefore used to measure differential gravity, without regard to the magnitude of gravity. Gravimeters, not gradiometers, are used in gravity surveys of the type described herein.
The typical relative gravimeter suspends a mass of known quantity with a spring-like device. An increase in gravity interacts with the known mass to slightly stretch or elongate the spring-like device. Conversely, a decrease in gravity allows the spring-like device to constrict slightly. In both cases, the position of the known mass changes by a slight amount due to the elongation or constriction of the spring-like device. The amount of physical displacement of the known mass is directly related to the magnitude of gravity at that location and time.
An absolute gravimeter is a much more technically sophisticated, delicate, expensive and physically larger instrument than a relative gravimeter, at least at the present time. In an absolute gravimeter, a mass of known quantity is positioned within a chamber which has been evacuated as much as possible to approximate a complete vacuum. A mechanism lifts the known mass and releases it to freefall within the chamber. A laser beam monitors movement of the free-falling mass, and an extremely accurate clock measures the time required for the mass to fall a specific distance or measures the speed of the free-falling mass at a specific time. By utilizing the distance and/or speed data, the magnitude of gravity acting upon the known mass at the time of the test is calculated. The gravity measurements from an absolute gravimeter are very precise, due principally to the technological sophistication of the device.
In a relative gravimeter, the spring-like device which suspends the known mass is susceptible to many influences that degrade the accuracy of the gravity measurements obtained. Changes in temperature and the age of the spring-like device can change its spring characteristics and hence change the displacement of the known mass. Changes in atmospheric pressure can also change its spring characteristics. The changes in the spring characteristics of the spring-like device are referred to as drift. Shocks caused by physical movement of the gravimeter can alter the at-rest position of the known mass. Changes in the at-rest position of the known mass are referred to as offset or tare. If these changes are not recognized and corrected, the resulting changes are interpreted incorrectly as influenced by gravity.
Because of their influences and responses, relative gravimeters are typically less reliable and less accurate than absolute gravimeters for measuring gravity. The effect of changes in temperature, pressure, drift and tare can mask any change in the magnitude of gravity, making it impossible to accurately measure gravity, particularly those small gravity changes resulting from the above-described changes in subsurface events.
In contrast, the gravity measurements obtained by using an absolute gravimeter are very accurate. However, the sensitivity and complexity of the absolute gravimeter has made it impossible, tedious, time-consuming and/or very difficult to employ an absolute gravimeter other than in a controlled scientific laboratory. Only recently have field-usable absolute gravimeters been developed, but such field-usable absolute gravimeters are expensive, in the neighborhood of US $300,000-500,000, which is about five or more times the price of a relative gravimeter.
The use of a field-usable absolute gravimeters in a typical gravity survey is further complicated by the requirement to obtain gravity measurements at a large number of survey points. A typical gravity survey may involve measuring gravity at many hundreds of different locations within a particular geographical or survey area. It may take as much as one-half of a day to set up a field-usable absolute gravimeter at each survey point. Even though movable, the sensitivity and fragility of a field-usable absolute gravimeter complicates its transportation from one survey point to another. Care must be taken to avoid damaging the delicate components of the absolute gravimeter when moving it. Consequently, the amount of time required to measure gravity at each of the many survey points of a typical survey area makes the use of absolute gravimeters impractical, prohibitive and almost impossible from both a logistical standpoint and a cost standpoint.
Due to the expense of a field-usable absolute gravimeter and the length of time required to obtain many gravity measurements using it, gravity surveys are typically conducted using a relative gravimeter. Gravity surveys conducted with a relative gravimeter typically involve a technique called “looping.” Looping involves obtaining gravity measurements at a starting point, at a series of intermediate points, and then again at the starting point. The sequence of gravity measurements begin and end at a single point, thereby creating a loop of gravity measurements.
The purpose of looping is to determine the amount of error in measurement that the relative gravimeter has suffered over the course of the measurement loop between the starting and ending gravity measurements. If the beginning and ending gravity measurements are different, as is typical of the case due to the above-described influences on relative gravimeters, the amount of the error determined by the difference in the beginning and ending gravity measurements at the starting point of the loop becomes a correction factor which should be applied to the intermediate gravity measurements. The correction factor is based on the assumption that the error changed in relation to time between the beginning and ending survey points in the loop. Consequently, the amount of correction applied to the first intermediate gravity measurement in the loop will generally be less than the amount of correction applied to the last intermediate gravity measurement in the loop. Of course, the underlying assumptions in applying the correction factor to all of the intermediate gravity measurements may itself also introduce some unknown inaccuracy in those corrected gravity measurements. However, correcting the intermediate gravity measurements results in considerably more accuracy than is available from the uncorrected relative gravity measurements.
Reducing the number of intermediate points within a survey loop reduces the number of gravity measurements that are affected by the inaccuracies inherent in relative gravity measurements in a looping survey. It is for this reason that a large number of loops are typically utilized in conducting a relative gravimeter gravity survey. Furthermore, to even better increase accuracy, the loops also include multiple survey points which have been measured in other loops, thereby resulting in an overlapping pattern of loops. The extent of looping increases the number of individual gravity measurements required and the amount of processing required to apply correction factors to the measurements obtained. Increasing the use of the relative gravimeter also increases the risks that the relative gravimeter will suffer errors due to the influences from the movement of the relative gravimeter required when executing the multiple loops.
Looping also increases the time and cost of conducting a gravity survey using relative gravimeters. Time is consumed because of the necessity to continually backtrack to previously-visited survey points in each loop. Many loops are required to complete the entire gravity survey. Because of the large number of gravity measurements required in a looping gravity survey, the relative gravimeter and other survey equipment experience additional wear and tear. Additional processing of the gravity measurement data is required because the data obtained from the many overlapping survey loops must be analyzed and correlated to derive the correction factors and apply those correction factors to obtain accurate gravity measurements.